The present invention relates to a measuring method and a measuring apparatus for trace ingredients in water, particularly for detecting and quantitatively determining trace silica, phosphorus or arsenic contained in a sample solution.
An analyzing method in which heteropolymolybdic acid is formed by using molybdic acid is conventionally commonly used for detecting silica, phosphorus or arsenic contained in a sample solution. That is, the molybdenum blue absorptiometry is adopted in the Japanese Industrial Standards (“JIS”), and comprises the steps of adding molybdic acid to the sample solution to form heteropoly acid (molybdenum yellow) which emits yellow color, and quantitatively determining the blue color (molybdenum blue) available upon reduction of heteropoly acid by the absorptiometry.
For the purpose of measuring silica, phosphorus or arsenic, there is available a method based on association of heteropolymolybdate ion and counter ion. A method which applies collecting ion association product into a membrane filter has been reported. This method comprises the steps of collecting the ion association product of molybdosilicate ion formed by use of Malachite Green, dissolving the thus-collected product into methyl cellosolve with the filter, and introducing the dissolved product into FIA (flow injection analysis) of trace silica in water (A. Sabarudin, et al.: Japan Society for Analytical Chemistry, The 50th Conference Lecture Abstract, p. 85 (2001)). A method of detecting phosphorus in the same manner as above by the use of Rhodamine B as a counter cation has also been reported. There is also available an arsenic detecting method comprising the steps of collecting ion association product of tetraphenylphosphonium-molybdoarsenate into a filter, and dissolving the thus-collected product into tetramethylammonium hydroxide to measure the same by ETAAS (electrothermal atomic absorption spectrometry).
In addition, the following analyzing methods based on association of heteropolymolybdate ion and counter ion have been reported: one is a method for measuring phosphorus by detecting the amount of decrease in the fluorescence intensity in the FIA process by utilizing the fact that ion association with molybdophosphate ion causes a decrease in the fluorescence intensity of Rhodamine 6G (Motomizu, et al.: Analytical Chemistry, 33:116 (1984); and another is a method for measuring phosphorus from the amount of decrease in the fluorescence of Rhodamine B in the FIA process by using Rhodamine B as a counter ion simultaneously with polyvinyl alcohol (Motomizu, et al.: The 63rd Analytical Chemistry Symposium Lecture Abstract, p. 7 (2002)).
Along with the recent general tendency toward a higher degree of integrity of semiconductor, there is an increasing demand for reducing the impurities concentration in ultrapure water for the semiconductor industry to the minimum level. Particularly, development of a method for rapidly monitoring trace silica on a ppb level contained in ultrapure water at a high sensitivity and at a high accuracy is urgently needed. From the point of view of preserving the water quality against eutrophication of lake water and seawater, or of solving the problem of ground water containing arsenic becoming more serious at various localities in Asia, the demand for rapid, high-sensitivity and high-accuracy measurement of phosphorus and arsenic, having important meaning in environmental measurement, on a ppb level is more apparent.
However, the molybdenum blue absorptiometry conventionally popularly used for measuring silica, phosphorus or arsenic requires improvements including the use of a long optical path cell while reducing blanks to overcome the insufficient sensitivity, and has many problems in cost and equipment scale.
The method which applies collecting ion association product through a filter is not practicable since it requires complicated operation and much time, and furthermore, it employs an organic solvent of which the adverse effect on living organisms is feared.
As compared with the molybdenum blue absorptiometry and the like, the measurement of phosphorus by the above-mentioned FIA process contains some contrivances for increasing the sensitivity and simplifying the process. This method is however to measure the amount of decrease in the fluorescence intensity of Rhodamine B corresponding to the phosphorus concentration in the sample solution when a reaction solution have reached the fluorescence detector through the prescribed flow path system. It requires a complicated setting of various parameters including setting of a reaction coil length and setting of driving conditions for the liquid transporting pump, and a decrease in the measuring accuracy (resolution) for a sample solution having ingredients to be measured of lower concentrations or a very slight difference in concentration is inevitable.